Fluorescent lighting has long been popular among consumers due to the increased efficiency of fluorescent lights over typical incandescent bulbs, and also due to the soft lighting effect provided by fluorescent bulbs.
A typical fluorescent bulb includes two tungsten filaments separated by a tube filled with ionizable gas, such as mercury vapor, with an argon or argon-neon starting gas, as well understood in the art. The filaments are usually coated with a special coating that is provided to supply additional electrons for ionization of the gas within the tube. When a very high voltage is supplied across the tube (i.e., from one filament to the other), electrons build up at the negative voltage filament until the gas within the tube is sufficiently ionized. Once the gas within the tube is ionized, the electrons flow through the gas to complete the circuit and produce electrical current flow through the tube, thereby illuminating the lamp.
Typically, the voltage levels across the bulb must be very high (e.g., several hundred volts) in order to ionize the gas sufficiently to produce current flow. Thus, a starting circuit having, for example, a step-up voltage transformer, is generally used as a ballast to provide the high voltage levels necessary for illuminating the lamps. Nevertheless, it is still advantageous to reduce the required starting voltage as much as possible. One method which is conventionally used to reduce the voltage necessary to produce current flow across the tube involves heating the filaments with a relatively low voltage. By heating the filaments, a greater supply of electrons is produced by the coated tungsten filaments so that it is possible to ionize the gas with a lower voltage across the tube.
The higher efficiency of fluorescent bulbs makes them ideal for applications where limited power is available, such as in the electrical system of recreational vehicles which often run off of a 12-volt battery. For example, fluorescent lamps are often used in recreational vehicles and the like. Many recreational vehicles have fluorescent lamps that turn on and off when the door opens and closes. If the owner uses the vehicle frequently enough, these lamps may be turned on and off up to 10,000 times a year. Unfortunately, currently available fluorescent lamps usually have an on/off life of 4,000-6,000 illuminations so that a recreational vehicle owner may have to replace these lamps as often as two or even three times a year.
The primary cause of failure for fluorescent light bulbs which are turned on and off frequently is burn-out of the filaments. The filaments generally burn out because the special coating on the filaments, which provides the main supply of electrons, is radiated off of the filaments and deposited onto the interior of the tube near the filaments. When the coating on the filaments becomes too thin, not enough electrons are provided to initiate current flow through the tube and the lamp will not illuminate. Typically, when the coating from one of the filaments is being radiated, a blue glow is observable at the end of the tube near the filament.
Radiation of the filament coating may be produced by a number of factors. Most notably, when a high voltage level is applied across the tube or the filament for an extended time period, a substantial quantity of the coating on the filament is likely to be radiated. Since the highest voltage levels across the tube and the filaments within a fluorescent light ballast circuit usually occur just prior to ignition (i.e., illumination) of the fluorescent lamp, it is preferable to minimize the start-up period as much as possible to prevent burn-out of the fluorescent lamps.